1. Field of the Invention
This invention relates to the preparation of an essentially cerium-free polymeric dispersion from a dispersion containing Ce (III) derived from Ce (IV) used to initiate free radical polymerization, to recovery of Ce (III) from polymeric dispersions and to regeneration of Ce (IV) from Ce (III) recovered from the polymeric dispersions.
2. Polymerization Products Containing Ce (III)
Ce (IV) has been widely used to initiate free radical polymerization of vinyl monomers. In the course of such reaction the Ce (IV) is reduced to Ce (III) which remains with the reaction products.
Mino et al., U.S. Pat. No. 2,922,768 disclose the use of Ce (IV) salts to induce vinyl polymerization on a wide variety of compounds such as those containing alcoholic hydroxy groups, ketones, mercaptans, amines, aldehydes, polyvinyl alcohol, cellulose and cellulosic materials, starch, partial ethers of starch and partial esters of starch. Some of Mino's products were polymeric dispersions (latices). The polymeric products were recovered by precipitation, i.e., destruction of the dispersion. Gugliemelli, et al., U.S. Pat. No. 3,377,302, prepared polymeric dispersions from granular or gelatinized starch polymerized with methyl acrylate using ceric ammonium nitrate to initiate the reaction. Separation of the polymer was accomplished by filtration. Similarly, Gugliemelli, et al., U.S. Pat. No. 3,425,971 separated the product obtained by using ceric ammonium nitrate to graft polymerize acrylonitrile onto starch by filtration. Jones, et al., U.S. Pat. No. 3,669,915, polymerized starch with 2-hydroxy-3-methacryloyloxypropyltrimethylammonium chloride. Solids were separated by centrifugation. D. A. Jones, et al., Die Starke 24, Jahrg. 1972/Nr 1. show starch and acrylonitrile polymerized with a ceric initiator. Here, also, separation was by centrifugation. Ichikawa, et al., Japanese Kokai No. 7668,641, graft polymerized polyvinylalcohol with acrylic monomers or chloroprene. The resultant aqueous solutions were passed over an ion exchange resin to remove acrylate monomer. In all cases except the latter, which involved a solution rather than an aqueous dispersion, methods which may have separated Ce (III) from the polymer resulted in destruction of the dispersion, if any.
3. Removal of Ce (III) from Resin
J. Mikler, et al. (CA, 64: 18921) have reported the removal of cerium (III) adsorbed on an ion exchange resin using lactic acid in caustic solution. This method appears to sequester the cerium in a soluble form, incapable of being reabsorbed by the resin, through formation of a complex ion of cerium and the lactic acid.
4. Oxidation of Cerium (III) to Ce (IV)
The +4 oxidation state of cerium is discussed in The Chemistry of the Lanthanides, T. Moeller (Selected Topics in Modern Chemistry) pp. 60-66, Reinhold Publishing Corporation, N.Y. (1963). Moeller discusses the oxidation of Ce (III) to Ce (IV) by chemical oxidizing agents such as S.sub.2 O.sub.8.sup.2- and O.sub.3 at page 61 where Moeller also notes that Ce (IV) is best prepared by electrolytic oxidation. Oxidation with oxygen at elevated temperatures through ignition in air of a thermally decomposable oxy anion is discussed at page 65 of this reference as is chemical oxidation with oxygen at ordinary temperatures. In this latter case Moeller notes that cerium (III) oxide or hydrous hydroxide slowly adsorbs atmospheric oxygen and ultimately yields the dioxide, CeO.sub.2. This reference also discloses that such oxidation is more rapid with alkaline hydrogen peroxide or sodium hypochlorite; that both bromate ion and permanganate ion oxidize Ce (III) to the dioxide in suitably buffered solutions; and that peroxy disulfate yields cerium (IV) under acidic conditions.